X-ray tubes are provided for generating a beam of X-rays. This X-ray beam may be transmitted through an object of interest and the transmitted X-rays may be detected using an X-ray detector thereby providing information about X-ray absorbing characteristics of the object of interest. For example, X-ray tubes may be applied in medical imaging for visualizing internal structures of a region of interest in a patient.
Recently, X-ray differential phase-contrast imaging (DPCI) has been developed for visualizing a phase information of coherent X-rays passing through a scanned object of interest. In addition to conventional X-ray transmission imaging, DPCI may determine not only absorption properties of the scanned object along a projection line but may also provide information about a phase-shift of transmitted X-rays. Thereby, valuable additional information usable e.g. for contrast enhancement, material composition information or dose reduction may be provided.
Principles of DCPI are discussed e.g. in WO 2011/070 521, US 2012/00 99 702 A1 and EP 173 10 99 A1. Generally, a standard X-ray source is provided for generating an X-ray beam. Between the X-ray source and the object of interest, a grating or grid having small openings is positioned. This grating is typically referred to as source grating G0. Portions of the X-ray beam transmitted through the openings of the source grating exhibit a certain degree of spatial optical coherence. Behind the object of interest, a second grating, typically named phase-shift grating G1, is placed and may operate as a beam splitter. A resulting interference pattern typically contains required information about a beam phase-shift in relative positions of its minima and maxima which are typically in an order of several micrometers. Since a common X-ray detector, typically having a resolution in the order of 150 μm, is not able to resolve such fine structures of minima and maxima, the interference pattern is generally sampled with a third grating, typically referred to as phase analyzer grating or absorber grating G2. The phase analyzer grating features the periodic pattern of transmitting and absorbing strips having a periodicity similar to the periodicity of the interference pattern. The similar periodicity generally produces a Moiré pattern behind the grating. This Moiré pattern has a much larger periodicity and is therefore detectable by a common X-ray detector. To obtain the phase-shift information, a shifting of one of the gratings, typically of the phase analyzer grating G2, laterally by fractions of a grating pitch is generally provided. Such lateral shifting is also referred to as phase stepping. The phase-shift information may be extracted from the particular Moiré pattern measured for each position of the analyzer grating.
However, it has been observed that non-optimum DPCI results may occur for example due to excessive inaccuracies in the positioning of the various gratings with respect to each other. The gratings, particularly the phase-shift grating and the phase analyzer grating in conventional DPCI systems, may have to be translated with respect to each other with a very high positional accuracy. Such high positional accuracy may be hard to obtain particularly e.g. in DPCI systems in which the X-ray tube and the X-ray detector together with the gratings are to be moved during X-ray examination such as e.g. in medical C-arm or CT X-ray imaging systems.